1. Field of the Invention
The present invention relates to an optical sending apparatus, an optical receiving apparatus, and an optical transmission system for supervising fault information and methods thereof. In particular, the present invention relates to an optical sending apparatus, an optical receiving apparatus, and an optical transmission system for supervising fault information at the time of sending, transmitting, and receiving of optical signals, respectively, in which signals at different bit rates coexist, and methods thereof.
2. Description of the Related Art
As the result of recent popularization of the Internet and the like, the amount of information that needs to be transmitted is increasing substantially. Against this backdrop, a wavelength-division-multiplexing transmission system is used. In the wavelength-division-multiplexing transmission system, multiple pieces of data are superimposed on optical signals having different wavelengths and the optical signals are multiplexed. Then, wavelength-multiplexed optical signals are transmitted through one optical fiber cable. As a result, it becomes possible to dramatically increase the amount of information transmitted with one optical fiber cable.
FIGS. 7 and 8 show a construction of an optical transmission system that performs the wavelength-division-multiplexing transmission described above.
An optical transmission system 1 shown in FIG. 7 includes an optical sending apparatus 2, an optical fiber 3, and an optical receiving apparatus 4. The optical sending apparatus 2 converts a first optical signal pa at a predetermined bit rate transmitted from a transmitter 5, which is an upstream apparatus, into a second optical signal pb having a required wavelength and optical output for long-haul optical communication. Then, the optical sending apparatus 2 sends out the second optical signal pb to the optical fiber 3 that is an optical transmission line. The optical fiber 3 transmits the optical signal pb. The optical receiving apparatus 4 receives the second optical signal pb passed through the optical fiber 3. Then, the optical receiving apparatus 4 converts the second optical signal pb into a third optical signal pc having a wavelength and optical output conforming to the standards of a receiver 6 that is a downstream apparatus. Finally, the optical receiving apparatus 4 outputs the third optical signal pc to the receiver 6. In this manner, communication between the transmitter 5 and the receiver 6 that are spaced apart from each other by along distance is made possible by the optical transmission system 1.
FIG. 8 is a block diagram showing a construction of the optical sending apparatus 2 in FIG. 7.
The optical sending apparatus 2 includes an optic-electric conversion circuit (O/E) 11, a clock and data signals regenerating circuit 12, branching circuits 13 and 14, an electric-optic converting circuit (E/o) 15, and a signal frame monitoring circuit 16. The optic-electric conversion circuit 11 converts the first optical signal pa inputted from the transmitter 5 into an electric signal ea. The clock and data signals regenerating circuit 12 regenerates a clock signal eb and a data signal ec from the inputted electric signal ea. The branching circuit 13 branches the clock signal eb into a signal for the electric-optic converting circuit 15 and a signal for the signal frame monitoring circuit 16. In a like manner, the branching circuit 14 branches the data signal ec into a signal for the electric-optic converting circuit 15 and a signal for the signal frame monitoring circuit 16. The electric-optic converting circuit 15 multiplexes the inputted clock signal eb and the data signal ec and then converts the multiplexed signals into a second optical signal pb.
In addition, the optic-electric conversion circuit 11 monitors signal disappearance (LOS: Loss of Signal) in the first optical signal pa. The signal frame monitoring circuit 16 monitors an out-of-synch state of frames (LOF: Loss of Frame) with reference to the inputted clock signal eb and the data signal ec.
It should be noted here that the transmitter 5 and the receiver 6 in FIG. 7 are pursuant to the standards of Synchronous Optical Network/Synchronous Digital Hierarchy (SONET/SDH), such as the specifications of STM-1 (bit rate: 155.52 Mbps) Also, there is a case where in place of the transmitter 5 and the receiver 6, for instance, a transmitter 5B and a receiver 6B pursuant to the standards of Gigabit Ethernet (GbE (registered trademark)) or a transmitter 5C and a receiver 6C pursuant to the fiber channel standards are used as shown in FIG. 7.
FIG. 9 shows examples of signal bit rates (Mbps) under the standards of applications. These signal bit rates are different from each other, so the optical sending apparatus 2 and the optical receiving apparatus 4 in FIG. 7 adopt specifications corresponding to the respective bit rates.
In the optical transmission system 1 described above, however, there arises the following problem.
In the optical transmission system 1, the bit rate of the first optical signal pa sent out from the transmitter 5 is determined to one kind. Therefore, when a signal at a different bit rate has been inputted from the transmitter 5, the signal frame monitoring circuit 16 detects the signal as an incorrect signal and halts sending out the second optical signal pb to the optical fiber 3. That is, there is a problem that the optical transmission system 1 is only capable of handling signals at a specific bit rate and is incapable of handling signals at different bit rates.
In view of this problem, an optical transmission system is also manufactured which is made capable of coping with transmission of signals at all bit rates by removing the function of the signal frame monitoring circuit 16. When the bit rate monitoring function is removed in this manner, however, even when a fault has occurred in the first optical signal pa from the upstream apparatus, it becomes impossible to detect the fault. Therefore, in this case, there arises a problem that signals containing faults are outputted to the downstream apparatus.
As a related technique other than the optical transmission systems described above, there is an optical relaying apparatus described in Related Art Document 1 “Japanese Patent Laid-Open No. 06-021960A, particularly see page 2 and FIG. 1”, for instance. The optical relaying apparatus converts an electric signal from a metal cable into an optical signal for an optical fiber transmission line and relays the optical signal. In addition, the optical relaying apparatus converts an optical signal from the optical fiber transmission line into an electric signal for the metal cable and relays the electric signal. The optical relaying apparatus is constructed so that bidirectional transmission is performed in this manner. Also, the optical relaying apparatus includes a signal-state detecting unit, a low-frequency signal generating unit, a switch, a frequency monitoring unit, and a test-data generating unit. The signal-state detecting unit detects whether a current state is a non-signal state. The low-frequency signal generating unit generates a specific signal that will not appear at the time of information transmission. The switch switches an input signal into an optical sending unit from a data signal to a specific signal. The frequency monitoring unit detects whether a received optical signal is the specific signal. The test-data generating unit sends out a test signal in accordance with output from the frequency monitoring unit. With the construction elements, when a fault has occurred in the optical fiber and the non-signal state has been detected at the signal-state detecting unit, the specific signal that will not appear at the time of information transmission is generated from the low-frequency signal generating unit. Then, when the specific signal has been detected at the frequency monitoring unit, the test signal is generated from the test-data generating unit. Next, the test signal is sent out to the optical fiber, to which the fault has occurred, by switching with the switch.
In the case of the optical relaying apparatus described in the Related Art Document 1, however, there is a problem that only faults concerning the non-signal state are monitored and it is impossible to perform monitoring of other faults.
In addition, in the optical transmission systems and the optical relaying apparatus, no method is proposed with which, when multiple kinds of faults have occurred, the faults are identified with ease.